1. Field of Invention
The present invention relates to an improved process for preparing fluoroaromatics or fluoropyridines by diazotization-fluorination of selected aromatic or heterocyclic amines in hydrogen fluoride. More particularly, the invention relates to processes in which a suitable amine substrate is diazotized with a diazotization agent in hydrogen fluoride to form a corresponding diazonium fluoride and the diazonium fluoride is decomposed to form a corresponding aromatic or heterocyclic fluoride. Specifically, the invention relates to the use of a solution of hydrogen fluoride containing ammonium ions as a medium in which to conduct the reaction.
2. Prior Art
Osswald et al, German Pat. No. 600,706 disclosed the conventional two-step process for diazotization-fluorination of primary aromatic amines utilizing sodium nitrite and anhydrous hydrogen fluoride. A solution was made of the amine substrate in hydrogen fluoride and sodium nitrite was added, the temperature during addition being held below about 10.degree. C until diazotization is complete. After completion of diazotization, the reaction mixture is refluxed (temperature 30.degree. C-40.degree. C) to decompose the diazonium fluoride, yielding nitrogen and the corresponding aromatic fluoride. Ferm and Vander Werf, J. Am Chem. Soc., 72 4809 (1950) expanded this technology to include other substrates. Shenk et al, U.S. Pat. No. 2,563,796, modified the process by utilizing gaseous nitrosyl chloride rather than sodium nitrite in order to obtain higher decomposition temperatures.
The Osswald et al and Ferm and Vander Werf teachings have been applied to numerous aromatic and heterocyclic amine substrates and their work-up procedures have been modified over the years to improve yields and facilitate product recovery.
Nevertheless, there is still a need for further improvements and efforts have been made to modify reaction conditions to obtain increased yields and/or further simplify the overall process. One such example is found in Misaki et al., Japanese patent publication No. 81330/74, published Aug. 6, 1974 pursuant Japanese application No. 126570/72, filed Dec. 15, 1972. In accordance with this teaching, improved yields were allegedly attained in a single step diazotization-decomposition by dissolving a diazotization agent, sodium nitrite, in excess anhydrous hydrogen fluoride (generating nitrous acid in HF), separately dissolving the diazotizable amine in hydrogen fluoride, then adding the sodium nitrite in HF to the amine in HF at temperatures in the range of 30.degree. C-50.degree. C to simultaneously form and decompose the diazonium fluoride to the aromatic fluoride.
It has now been found that whether one employs the conventional two-step process or its modifications or the one-step process the presence of ammonium ions in the hydrogen fluoride solution or medium, surprisingly and unexpectedly, enhances yields beyond those obtainable when diazotization is conducted in HF alone, and moreover, permits the successful diazotization-fluorination in HF of substrates which with HF alone give principally tars.